Devices and methods to control dynamic audio range in boosted audio systems

ABSTRACT

A controller regulates the voltage delivered to the load and current drawn from the battery in an audio system depending on ripple in the battery voltage which is input to the controller to allocate power for audio playback. Regulation maximizes available headroom while avoiding audio clipping. The effect of internal battery and external parasitic resistance (ESR) on ripple is compensated by an iterative process. ESR is rapidly increased whenever the minimum of the battery voltage input to the controller falls below a clipping threshold and slowly decreased whenever such voltage exceeds such threshold and the audio is under compression. A limiter allocates power to utilize more of the available audio headroom. A de-emphasis filter in each audio signal path compensates for capacitive ripple in the battery voltage input to the controller. As the frequency of the audio input changes, the filter(s) allow frequency-dependent power/current regulation to fill the full audio range without distortion.

FIELD OF DISCLOSURE

This disclosure relates generally to devices and methods that controland improve dynamic audio range in boosted audio systems, and moreparticularly to devices and methods that enable full use of theavailable audio range while limiting clipping, and that enable batterycurrent regulation and preserve battery life.

BACKGROUND

A boosted audio playback system provides an amplified output voltage viaa boost circuit to drive speakers. The voltage input to the boostcircuit is provided by the power supply, e.g., battery, of the system.The voltage at the input of the boost circuit typically suffers fromripple due to internal battery resistance and external parasiticresistance (a combination of which is also known as external seriesresistance or ESR), and capacitive decoupling. Ripple also depends onthe instantaneous current drawn from the battery to power the load. Ifthe current drawn from the battery is too high, then the ripple tends tocause the battery voltage at boost circuit input to drop significantly,resulting in the instantaneous input power being less than the outputload power. This results in clipping. If the current drawn from thebattery is too low, then the ripple too will be low, which results inthe instantaneous input power being greater than the output load power.This means that headroom (upper portion of the audio range) is availableand that target output power could have been further increased. Theresulting effect is reduced output loudness. Thus, regulating thecurrent drawn from the battery is an important consideration in audiosystems.

Conventional approaches apply a low pass filter in the battery voltagesensing path, and thus only consider the DC component of the ripple.Such approaches do not consider the instantaneous AC drop, which affectsthe boost circuit of the system. Even adding a margin to account for theAC ripple would not solve the problem, since the AC ripple depends onESR which depends on ambient conditions such as temperature. Also, thedecoupling capacitor causes the ripple to be frequency dependent. Thus,such conventional approaches fail to accurately compensate for theripple, as they inevitably either over compensate (resulting in lowerloudness) or under compensate (resulting in clipping and brown-out). Abetter solution is thus desirable.

SUMMARY

In accordance with an example, a controller comprises a voltageestimator having inputs to receive multiple input voltage valuesobtained from an input voltage signal and an output at which a voltagesignal (estimated VBAT) representing an estimate of the true inputvoltage signal is output. The controller also comprises a currentregulator having a first input to receive one of the multiple inputvoltage values, a second input coupled to the output of the voltageestimator to receive the estimated VBAT, and an output at which acalculated current limit signal is output. The current regulator tracksan estimated resistance and controls the estimated resistance to be aspecific value based on the input voltage value received by the currentregulator. The current regulator calculates the current limit signalbased on the estimated VBAT, a clipping voltage threshold and thespecific value of the estimated resistance.

In accordance with an example, an audio system comprises a voltageestimator that generates a voltage signal (estimated VBAT) representingan estimate of true voltage input to the audio system based on multipleinput voltage values; a current regulator that tracks an estimatedresistance and controls the estimated resistance to be a specific valuebased on one of the multiple input voltage values that is received bythe current regulator, the current regulator calculating a current limitsignal based on the estimated VBAT, a clipping voltage threshold and thespecific value of the estimated resistance; a filter disposed in anaudio signal path, the filter having an output at which a filtered audiosignal is output; and an audio limiter having a first input at which thefiltered audio signal is received, a second input at which a voltagelimit signal is received, a third input at which a load resistance valuedetermined from an audio output signal is received, and a fourth inputat which an input power signal is received.

In accordance with an example, a method comprises measuring an inputvoltage signal to a controller of an audio system in each of multipleperiods of time to calculate, for each period of time, a maximum valueof the input voltage signal (maximum value), an average value of theinput voltage signal (average value), and a minimum value of the inputvoltage signal (minimum value); outputting, by the controller, an outputvoltage for each period of time representing an estimate of the truevalue of the input voltage signal based on the maximum value for thatperiod of time and the average of multiple maximum values sampled duringthat period of time; estimating a resistance for each period of timebased on the minimum value for that period of time; and calculating anoutput current of the controller for each period of time based on theoutput voltage for that period of time and the estimated resistance forthat period of time.

In accordance with an example, an audio system comprises a limiterhaving a filtered audio input at which a filtered audio signal isreceived, a voltage limit input at which a voltage limit signal isreceived, and a gain output at which a gain signal, calculated based onthe filtered audio signal and the voltage limit signal, is output. Thelimiter includes a smoothing filter configured to estimate the amplitudeof the filtered audio signal and output a signal representing asmoothened estimate of the amplitude of the filtered audio signal(smoothened estimate signal).

These and other features will be better understood from the followingdetailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the disclosure may be understood from the following figurestaken in conjunction with the detailed description.

FIG. 1 is a schematic diagram of an example controller including anexample battery voltage estimator and an example current regulator.

FIG. 2 is a schematic diagram of components of an example audio systemthat may be controlled by the controller of FIG. 1.

FIG. 3 is a schematic diagram of an example battery system coupled to anexample audio system.

FIG. 4 is a schematic diagram of an example power threshold calculator,such as that shown in FIG. 2.

FIG. 5 is a schematic diagram of example limiters and other componentsof the audio system of FIG. 2.

FIG. 6 is a flow diagram of example operation of a controller, such asthat shown in FIG.

FIG. 7 is a flow diagram of example operation of a first stage limiterin a two-stage limiter assembly.

FIG. 8 is a flow diagram of example operation of a second stage limiterin a two-stage limiter assembly.

The same reference numbers are used in the drawings to designate thesame or similar (structurally and/or functionally) features.

DETAILED DESCRIPTION

Specific examples are described below in detail with reference to theaccompanying figures. These examples are not intended to be limiting.The objects depicted in the drawings are not necessarily drawn to scale.

In an example, audio is controlled based on estimation of true batteryvoltage (VBAT) from the raw battery voltage (Raw VBAT) and calculationof a target current limit (ILIM). By controlling the audio based onestimated VBAT and calculated ILIM, the target current drawn from thepower supply, e.g., battery, is regulated. Calculation of ILIM is basedon: estimated VBAT and adjustments made to ESR, which adjustments arebased on Raw VBAT and a clipping voltage threshold (VIER), whichrepresents a clipping or brown-out threshold. A de-emphasis filterdisposed in the audio signal path compensates for frequency-dependent,e.g., capacitive, ripple in Raw VBAT.

In an example, an audio limiter assembly allocates power in an audiosystem based on estimated VBAT and calculated ILIM, and includes ade-emphasis filter to compensate for capacitive ripple in Raw VBAT. Inan example, a VBAT estimator estimates true battery voltage from RawVBAT at a low ripple/silence condition. In an example, a currentregulator calculates the current (ILIM) based on (estimatedVBAT−V_(THR))/estimated ESR, where estimated ESR is rapidly increased ifestimated VBAT falls below V_(THR), and if not, is slowly decreased ormaintained the same depending on another condition. Estimated ESR may bereset to a default value after a prolonged period of silence.

Thus, calculated ILIM dynamically adapts to changes in system conditionsby tracking VBAT and ESR, while a de-emphasis filter in the audio signalpath compensates for frequency-dependent, e.g., capacitive, ripple. Thisensures that the power allocated to audio playback will be the result ofthe instantaneous input raw battery voltage (Raw VBAT) being aboveV_(THR) and be equal (or substantially equal) to a target output loadpower, which depends on estimated VBAT and ILIM. Such control of audiopower results in battery current regulation, increases (e.g., maximizes)sound pressure level (SPL) and reduces (e.g., minimizes) clipping. Sincethe audio power regulation adapts to changing VBAT and ESR, the currentdrawn from the battery scales with system conditions and isautomatically adjusted to the appropriate amount. This improves batterylife.

FIG. 1 illustrates a controller 100 having left (L) and right (R)battery signal sensors 102 and 104 including L and R battery signalinputs, respectively. The L and R battery input signals are eachdesignated as VBAT|_(device), such that there is L channelVBAT|_(device) and R channel VBAT|_(device). Each of battery signalsensors 102, 104 may include an analog-to-digital converter (ADC) todigitize the incoming analog battery signal. The sensed L and R devicevoltages are input to terminals of measurement logic 106 which averagesthe received voltages to generate a raw battery voltage signal (RawVBAT). In another example, a single device voltage (VBAT|_(device)) maybe input to measurement logic 106, in which case averaging is notperformed, in which case Raw VBAT is the same as VBAT|_(device).

Controller 100 applies a control algorithm to generate/update estimatedVBAT and calculate ILIM over multiple sampling/calculation intervals,which are then used to regulate audio. The samples input to measurementlogic 106 are designated by the index m, and the time interval betweentwo successive indices m and (m−1) is designated as the sample interval.An example sample interval can be 10.4 μs (corresponding to a samplingrate of 96 KHz). This is the typical sampling rate with which the ADCdigitizes the VBAT|_(device) for L and R channels, although othersampling rates may be used. Additionally, measurement logic 106 mayupsample the received signals from the ADC to improve the accuracy ofthe received samples and may reduce the time interval between successiveindices m and (m−1). In an example, measurement logic 106 may upsampleby a factor of 4, in which case the sample interval for successiveindices m and (m−1) is 2.6 μs (corresponding to a sampling rate of 384KHz). Calculations using a collection of samples can be updated using alarger time interval, e.g. a window of time, that is designated by theindex n, and the time interval between two successive indices n and(n−1) is designated as the calculation interval. An example calculationinterval can be 10 ms corresponding to samples m=0, 1, . . . N−1, whereN=block size corresponding to the calculation interval and can be equalto 960 (for sampling rate=96 KHz) or equal to 3840 (for a samplingrate=384 KHz). Within a calculation interval, measurement logic 106processes Raw VBAT (or upsampled version of Raw VBAT), which has ripple,to determine the signal's maximum value Max VBAT(n)=(max(RawVBAT(m))|_(m=0, 1, . . . , N-1)), average value

$\left. {{{Avg}{{VBAT}(n)}} = {{{avg}\left( {{Raw}{VBAT}} \right)} = {\sum_{m = 0}^{N - 1}\frac{{Raw}{{VBAT}(m)}}{N}}}} \right),$

and minimum value Min VBAT(n)=(min(Raw VBAT(m))|_(m=0, 1, . . . , N-1)).For each calculation interval, measurement logic 106 outputs a MaxVBAT(n) value, an Avg VBAT(n) value, and a Min VBAT(n) value.

VBAT estimator 108 has inputs to receive Max VBAT(n) and Avg VBAT(n)from measurement logic 106. Based on those input values, VBAT estimator108 estimates VBAT for each calculation interval, i.e., VBAT(n). Foreach calculation interval, VBAT estimator 108 determines an intermediatevalue (Max VBAT Temp(n)) based on the difference between Max VBAT(n) andAvg VBAT(n). When the difference between Max VBAT(n) and Avg VBAT(n) isless than a silence voltage threshold, Max VBAT Temp(n) is equal to MaxVBAT(n); otherwise, Max VBAT Temp(n) remains the same, i.e., Max VBATTemp(n)=Max VBAT Temp(n−1). In an example, the silence voltage thresholdis 10 mV. Based on Max VBAT Temp(n), VBAT estimator 108 may determineVBAT(n) to be the average of Max VBAT Temp(n) and those from a certainnumber of previous calculation intervals. The total number of Max VBATTemp values used to calculate the average is denoted by M, and in such

${{VBAT}(n)} = {\sum_{k = 0}^{M - 1}{\frac{{Max}{VBAT}{{Temp}\left( {n - k} \right)}}{M}.}}$

M may be 5, for example. VBAT estimator 108 outputs VBAT(n) for eachcalculation interval. Thus VBAT(n) is an estimate of VBAT, which is thetrue battery voltage.

Each time VBAT is measured (i.e., each time the true battery voltage isestimated), ILIM is changed (i.e., recalculated), since ILIM is directlyrelated to estimated VBAT (i.e., ILIM(n)=(VBAT(n)−V_(THR))/ESR(n)), evenif ESR(n) remains constant from one calculation to the next, to ensurethat the battery voltage ripple is within V_(THR). The changing of ILIMis described in more detail below in the context of ESR estimation,which may also be changed depending on certain conditions.

Current regulator 110 includes ESR control logic 112 and a current limitcalculator 114. ESR control logic 112 receives Min VBAT(n) frommeasurement logic 106. For each calculation interval n, ESR controllogic 112 determines whether, and by what amount, to adjust estimatedESR based on a comparison of Min VBAT(n) with V_(THR) and whether or notthe audio is under compression. At each calculation interval n, ESRcontrol logic 112 determines whether Min VBAT(n) is less than V_(THR).If Min VBAT(n)<V_(THR), ESR(n), which is an estimate of ESR, isincreased in a fast attack manner; that is, ESR(n)=ESR(n−1)+an attackamount of resistance, which may be 0.05Ω. This ensures that ILIM iscalculated and audio power accordingly regulated so that the batteryvoltage ripple is within V_(THR). If Min VBAT(n)≥V_(THR)+a threshold andthe audio is compressed, ESR(n) is reduced in a slow release manner;that is, ESR(n)=ESR(n−1)−a decay amount, which may be 0.01Ω. Thisensures that available headroom is recovered when ESR reduces due tochange in ambient conditions. In an example, V_(THR) may be 3 V and thethreshold may be 100 mV. If Min VBAT(n)≥V_(THR)+threshold and the audiois not under compression, estimated ESR remains the same; that is,ESR(n)=ESR(n−1). This means that even if ESR has decreased due to changein ambient condition(s) there is already enough audio headroom availableso no ESR adaptation is required. ESR control logic 112 transmits ESR(n)to current limit calculator 114, which also receives VBAT(n) from VBATestimator 108. For each calculation interval, current limit calculator114 calculates the current (ILIM(n)) according to the equation:ILIM(n)=(VBAT(n)−V_(THR))/ESR(n). In an example, if Max VBAT(n) isapproximately equal to Avg VBAT(n) for a period of time, e.g., prolongedsilence, for example, 5 minutes), then ESR(n) may be reset to an initialvalue as a safety mechanism, which action may be triggered by a controlsignal from VBAT estimator 108 indicating such condition.

Thus, controller 100, through the control algorithm, repeatedlygenerates/updates VBAT(n) and calculates ILIM(n) based on systemconditions. VBAT(n) and ILIM(n) are then used by other components of anexample audio system 200, which is shown in FIG. 2, to regulate audio.

Whether the audio is under compression can be determined by comparingthe left channel audio out signal (AUDIO OUT L) with the left channelaudio input signal and comparing the right channel audio out signal(AUDIO OUT R) with the right channel audio input signal (see FIG. 2). Ifthe amplitude of either AUDIO OUT L or AUDIO OUT R is less than thecorresponding amplitude of audio input signal, then the audio is undercompression due to the control algorithm. However, if MinVBAT(n)≥V_(THR)+threshold, then there is excess headroom available toreduce (or remove) the compression. So, in such scenario, the ESR isreduced. As the ESR reduces, the ILIM increases, which means that morepower (VBAT×ILIM) is available for audio and this ensures thatcompression is reduced or removed, which helps in increasing theloudness of the output audio. On the other hand, if the amplitude ofboth AUDIO OUT L and AUDIO OUT R is equal to the amplitude of thecorresponding audio input signal, then the audio is not undercompression due to the control algorithm. In this case, even if MinVBAT(n)≥V_(THR)+threshold, and there is excess headroom available, thereis no need to reduce ESR, increase ILIM and increase available power,because the current available power is sufficient to drive the audiowithout any compression.

Example audio system 200 of FIG. 2 comprises an audio signal input,which in the illustrated example, includes a left channel (L) audiosignal input 202 and a right channel (R) audio signal input 204 fromwhich the left and right channel audio signals are respectively input.The output of L audio signal input 202 is coupled to a left channelfilter 206, e.g., a de-emphasis filter, and a look ahead delay 208. Theoutput of R audio signal input 204 is coupled to a right channel filter210, e.g., a de-emphasis filter, and a look ahead delay 212. De-emphasisfilters 206 and 210 compensate for frequency-dependent, e.g.,capacitive, ripple in the audio signal path. De-emphasis filters 206 and210 de-emphasize higher frequency components of the audio signal.

Example audio system 200 further comprises a power threshold calculator214, which has a voltage input at which VBAT(n) is received and acurrent input at which ILIM(n) is received. Power threshold calculator214 also includes inputs to receive left and right channel resistancevalues (Res L(n) and Res R(n)) indicative of the time-varyingresistances in the left and right audio output signals delivered to theload, i.e., left and right speakers 216 and 218, respectively. Asdescribed in more detail below in connection with FIG. 4, for eachcalculation interval n, power threshold calculator 214 calculatesvoltage limit signals (Voltage Limit L(n) and Voltage Limit R(n)) basedon VBAT(n), ILIM(n) and the resistances of the output audio signals (ResL(n) and Res R(n)). Res L(n) is considered in calculating Voltage LimitL(n), and Res R(n) is considered in calculating Voltage Limit R(n).

Example power threshold calculator 214 includes an input powercalculator 214 a which has inputs at which VBAT(n) and ILIM(n) arereceived. From these signals and a scale factor, input power calculator214 calculates an input power total (Input Power Total(n)), which is oneof the inputs to limiter assembly 220. In an example, the scale factormay be 0.85.

Limiter assembly 220, which is shown and described in more detail withrespect to FIG. 5, also receives left (L) and right (R) filtered audiosignals from filters 206 and 210, respectively, the Voltage Limit L(n)and Voltage Limit R(n) voltage signals from power threshold calculator214, as well as the Res L(n) and Res R(n) values, which are determinedand updated by sensor & measurement components 222 and 224,respectively, at each calculation interval n.

Current/voltage sensor & load resistance measurement component 222 hasan input coupled to the L audio out channel, an ADC to sense anddigitize the current and voltage in that output signal, and circuitry todetermine the load resistance (Res L(n)) based thereon. Similarly,current/voltage sensor & load resistance measurement component 224 hasan input coupled to the R audio out channel, and ADC to sense anddigitize the current and voltage in that output signal, and appropriatecircuitry to determine the load resistance (Res R(n)) based thereon. ResL(n) and Res R(n), which are the same values input to power thresholdcalculator 214, are used by limiter assembly 220 to control total powerand make use of any available headroom.

Limiter assembly 220 controls the gain based on the left and rightchannel filtered audio signals received from filters 206 and 210,respectively, based on Voltage Limit L(n), Voltage Limit R(n), InputPower Total(n), Res L(n) and Res R(n). Limiter assembly 220 outputs leftand right channel gain signals to multipliers 236 and 238, respectively.Multiplier 236 also receives the output of look ahead delay 208, andmultiplier 238 also receives the output of look ahead delay 212.

Multiplier 236 multiplies the output of look ahead delay 208 with leftchannel gain signal from limiter assembly 220 to generate an audiosignal that is input to adder 246, which also receives pilot tone 242.Multiplier 238 multiplies the output of look ahead delay 212 with theright channel gain signal from limiter assembly 220 to generate an audiosignal that is input to adder 248, which also receives pilot tone 244.Based on their respective inputs, adders 246 and 248 generate left andright channel audio out signals, respectively. Additional description ofthis processing is described in connection with FIG. 5.

Audio system 200 further comprises a boost circuit for each speaker. Tothat end, in the illustrated example, boost circuit 226 is coupled to Lspeaker 216, and boost circuit 228 is coupled to R speaker 218. Eachboost circuit may be part of an amplifier, e.g., a Class D amplifier,indicated by the dashed enclosure. Each boost circuit 226 and 228 has avoltage input at which VBAT|_(device) is received and a current input atwhich ILIM(n) is received, as well as an output coupled to speaker 216and 218, respectively. The output drives power for speakers 216 and 218to play Audio Out L and Audio Out R, respectively. Each boost circuit226 and 228 may also include, or have coupled to it, a boost inductor,as shown in FIG. 3. An output capacitor 232 is disposed in the powertransmission path between boost circuit 226 and L speaker 216. Likewise,output capacitor 234 is disposed in the power transmission path betweenboost circuit 228 and R speaker 218. The effect of the output capacitors232 and 234 in the context of the control algorithm performed bycontroller 100 is described in more detail below in connection with FIG.3.

FIG. 3 is a schematic diagram showing a battery system 300 coupled to anaudio system 302, e.g., audio system 200, which includes a boost circuit304 and a load, e.g., speaker 306, coupled to the output of boostcircuit 304. Boost circuit 304 may correspond to either of the boostcircuits 226 or 228 of FIG. 2, and speaker 306 may correspond to eitherof speakers 216 or 218 of FIG. 2. Audio system 302 also includes anoutput capacitor 308 coupled to the output of boost circuit 304. Outputcapacitor 308 may correspond to either of the output capacitors 232 or234 of FIG. 2.

Battery system 300 includes a battery 310 which includes internalresistance. There is also external parasitic resistance. The internalbattery resistance plus the external parasitic resistance represents theESR, which is identified by reference numeral 312 in FIG. 3. Asdescribed above, current regulator 110 tracks and considers ESR 312 incalculating ILIM(n). The effect of decoupling capacitance caused bydecoupling capacitor 314 is also considered in de-emphasis filters 206and 210 of audio system 200 in FIG. 2. Decoupling capacitor 314 isinterposed between the two resistive components of ESR 312.

Boost circuit 304 has one terminal which is coupled to a boost inductor316 to receive VBAT|_(device); the same terminal is coupled to an ADCwhose other terminal is coupled to measurement logic 106 shown inFIG. 1. The ADC functions similarly to the ADC in each of sensors 102and 104. Boost inductor 316 draws current from battery 310 and storespower during the charging phase. The power stored by inductor 316depends on IBAT, which is the current drawn from battery 310 andVBAT|_(device), which is the instantaneous voltage at voltage inputterminal of boost circuit 304. Inductor 316 then transfers power tooutput capacitor 308 in the discharge phase, and output capacitor 308drives load 306 to play audio through an amplifier, e.g., a Class Damplifier. Boost circuit 304 uses the received ILIM(n) to program thepeak current limit of boost circuit 304. Boost inductor 316 chargesuntil a peak current limit is reached. That is, audio is regulated suchthat boost inductor 316 charges using a current that is less thanILIM(n). In an example, this peak current limit may be dynamicallyupdated based on ILIM(n) calculated by current limit calculator 114.

The control algorithm, described above and indicated by referencenumeral 320 in FIG. 3, regulates audio power based on VBAT|_(device) (orRaw VBAT). That is, the true battery voltage VBAT is estimated, and theaudio is regulated such that any transient drop in VBAT|_(device) belowV_(THR) is quickly corrected by rapidly increasing the estimate of ESR312 to elevate VBAT|_(device) (or Raw VBAT) above V_(THR). Ignoring thefrequency component due to decoupling capacitor 314, VBAT|_(device) isequal to the true battery voltage less the product of ESR 312 and IBAT,i.e. VBAT|_(device)=VBAT−ESR×IBAT. With ILIM defined as(VBAT−V_(THR))/ESR, as described above, if IBAT is less than or equal toILIM, then VBAT|_(device) is greater than or equal to V_(THR). Underthis condition, when IBAT=ILIM, VBAT|_(device)=V_(THR), and thiscorresponds to the maximum instantaneous power delivered by battery 310which is equal to the product of V_(THR) and ILIM i.e.,Power(VBAT|_(device))=V_(THR)×ILIM. To ensure IBAT is less than or equalto ILIM, audio should be regulated such that audio power is less than orequal to the product of V_(THR) and ILIM.

However, the algorithm regulates audio voltage such thatPower(Audio)≤VBAT×ILIM (not V_(THR)×ILIM), i.e., it overdrives thesystem because VBAT>V_(THR). Because audio is a dynamic signal, therewill be times of low audio levels when Power(Audio)≤V_(THR)×ILIM, i.e.,Power(VBAT|_(device)) and there will be times of high audio levels whenPower(VBAT|_(device))≤Power(Audio)≤VBAT×ILIM. For low audio levels whenPower(Audio)≤Power(VBAT|_(device)) the power delivered by battery 310 issufficient to drive the audio power. However, for high audio levels,when Power(VBAT|_(device))≤Power(Audio)≤VBAT×ILIM, the power deliveredby the battery is insufficient to drive the audio power, indicating thatthere is a power shortfall. This shortfall is compensated by outputcapacitor 308, which is charged when excess power is available, i.e.,when Power(VBAT|_(device)) is greater than Power(Audio), i.e., inperiods of low audio levels. Depending on how much shortfall outputcapacitor 308 can accommodate, the scale factors (Scale L, Scale R andScale Total) of power threshold calculator 214 (shown in FIG. 4) can betuned such that Power(Audio) does not exceed the sum ofPower(VBAT|_(device)) and Power(Output Capacitor) and there is noclipping of the output audio signal. The scale factors are described inmore detail in the context of the description of FIG. 4, which shows anexample power threshold calculator 214.

VBAT|_(device) has ripple, which depends on ESR 312, the capacitance ofdecoupling capacitor 314 and IBAT. Decoupling capacitor 314 acts as alow pass filter to reduce the AC component of the ripple ofVBAT|_(device) at higher frequencies, which means that more power can beaccommodated at higher frequencies. This accommodation is realized byde-emphasis filters 206 and 210 of audio system 200 of FIG. 2. Thesefilters 206 and 210 are low pass filters modeled according to decouplingcapacitor 314, and they de-emphasize the audio signal level at highfrequencies before sending it to limiter assembly 220 of FIG. 2. Due tothe de-emphasis effect, the gain control of limiter assembly 220 at highfrequency is less, meaning that the audio output will have more power.An example cut-off frequency of de-emphasis filters 206 and 210 is 43KHz. The peak level of VBAT|_(device) also reduces at higher frequenciesbecause of the DC component of the ripple. Thus, VBAT is estimatedduring low ripple, silence condition, i.e., when the ripple is less than10 mV.

FIG. 4 is a schematic diagram of power threshold calculator 214 of FIG.2. Power threshold calculator 214 includes a multiplier component 402,which has a voltage input and current input. VBAT(n) is received at thevoltage input, and ILIM(n) is received at the current input. Powerthreshold calculator 214 outputs two voltage signals: a left channelvoltage limit signal (Voltage Limit L(n)) and a right channel voltagelimit signal (Voltage Limit L(n)), which are updated every calculationinterval n. Power threshold calculator 214 also outputs a third signal:Input Power Total(n), which represents the available power and is alsoupdated every calculation interval n.

Based on the voltage and current signal inputs, multiplier 402calculates an intermediate power signal (Power(n)) that is delivered tomultiplier 408 and multiplier 412 for the left and right channels,respectively.

In the illustrated example, Power(n) output from multiplier 402 ismultiplied by a scale factor for the left channel (Scale L) 406 usingmultiplier 408 and is multiplied by a scale factor for the right channel(Scale R) 410 using multiplier 412. Scale L and Scale R typicallyreflect the efficiency of the system (including the boost circuits 226and 228 and corresponding Class D amplifiers) and the capacity of outputcapacitor 308 to accommodate extra power. The scale factor of Scale L406 and Scale R 410 may each have a value of 0.85 (85%), as an example.

Multiplier 408 thus scales the intermediate power signal received frommultiplier 402 to generate an input power L signal (Input Power L(n)).Similarly, multiplier 412 scales Power(n) received from multiplier 402to generate an input power R signal (Input Power R(n)). Input Power L(n)and Input Power R(n) are input to multipliers 414 and 416, respectively,which also receive resistance values Res L(n) and Res R(n). The productof Input Power L(n) and Res L(n) is input to a square root functionalcomponent 418 which computes the square root of such product to generatethe limit L voltage signal. The product of Input Power R(n) and Res R(n)is input to a square root functional component 420 to generate the limitR voltage signal. Thus, Voltage Limit L(n=√{square root over (InputPower L(n)×Res L(n))}, and Voltage Limit R(n)=√{square root over (InputPower R(n)×Res R(n))}.

Input power calculator 214 a of power threshold calculator 214 includesa multiplier 422 that receives the intermediate power signal (Power(n))from multiplier 402 and a total scale factor (Scale Total) 424 by whichthe intermediate power signal is scaled to generate Input PowerTotal(n). The scale factor of Scale Total 424 may be, for example, 0.85(85%).

FIG. 5 is a schematic diagram of limiter assembly 220 along withupstream and downstream components for context. In a multi-channelsystem, limiter assembly 220 includes two first stage limiters: leftlimiter (Limiter L) 502 and right limiter (Limiter R) 504 to providemore headroom in such system. Limiter L 502 has an audio input at whichthe filtered audio signal (ln_(L)(k)) is received from L filter 206 anda voltage input at which Voltage Limit L(n) is received from powerthreshold calculator 214. Similarly, Limiter R 504 has an audio input atwhich the filtered audio signal (ln_(R)(k)) is received from R filter210 and a voltage input at which Voltage Limit R(n) is received frompower threshold calculator 214. The audio samples are designated by thetime index k, and the time interval between two successive indices k and(k−1) is designated as the audio sample interval. An example audiosample interval can be 20.8 μs (corresponding to a sampling rate of 48KHz).

Limiter L 502 receives the left channel filtered audio signal(In_(L)(k)) and compares its absolute value (|In_(L)(k)|) with theestimate of the peak in the immediate previous audio calculationinterval (In_(est_L)(k−1)) to estimate the amplitude of In_(L)(k), suchestimate denoted by In_(est_L)(k). If In_(est_L)(k−1)<|In_(L)(k)|, thenIn_(est_L)(k)=|In_(L)(k)|; otherwise, In_(est_L)(k)=In_(est_L)(k−1)×adecay factor. Similarly, limiter R 504 receives the right channelfiltered audio signal (In_(R)(n)) and compares its absolute value(|In_(R)(k)|) with the estimate of the peak in the immediate previousaudio calculation interval (In_(est_R)(k−1)) to estimate the amplitudeof In_(R)(k), such estimate denoted by In_(est_R)(k). IfIn_(est_R)(k−1)<|In_(R)(k)|, then In_(est_R)(k)=|In_(R)(k)|; otherwise,In_(est_R)(k)=In_(est_R)(k−1)×a decay factor. The decay factor may be0.9993 (corresponding to a decay time constant of 30 ms for an audiosampling rate of 48 KHz), for example. To prevent or minimizedistortion, limiter L 502 and limiter R 504 may include respectivesmoothing filters 502 a and 504 a through which In_(est_L)(k) andIn_(est_R)(k) are respectively passed to generate signalsFilter[In_(est_L)(k)] and Filter[In_(est_R)(k)], respectively. Limiter L502 outputs the Filter[In_(est_L)(k)], and Limiter R 504 outputs theFilter[In_(est_R)(k)] which are smoothened estimates of the amplitude ofIn_(L)(k) and In_(R)(k), respectively. Limiter L 502 calculates andoutputs a gain signal (Gain1L(k)) according to the formula:

${{MIN}\left( {1,\frac{{Voltage}{Limit}{L(n)}}{{Filter}\left\lbrack {{In}_{est_{L}}(k)} \right\rbrack}} \right)},$

and limiter R 504 calculates and outputs a gain signal (Gain1R(k))according to the formula:

${{MIN}\left( {1,\frac{{Voltage}{Limit}{R(n)}}{{Filter}\left\lbrack {{In}_{est_{R}}(k)} \right\rbrack}} \right)}.$

Each smoothing filter 502 a and 504 a ensures that the correspondinggain signal does not have abrupt transitions and prevents audioartifacts. To preserve audio balance between the two channels,especially for balanced speakers which respond uniformly to the sameaudio frequency, a common gain may be used, which is the minimum ofGain1L(k) and Gain1R(k). Thus, component 506 selects and outputs theminimum of Gain1L(k) and Gain1R(k)) as Gain1(k). For unbalancedspeakers, for example, a loudspeaker and a receiver speaker, or a wooferspeaker and a tweeter speaker, which do not respond uniformly to thesame audio frequency, component 506 may be omitted, and in such casesGain1L(k) and Gain1R(k) is used for subsequent calculations.

Multipliers 512 and 514 each receive the common gain signal Gain1(k) forbalanced speakers. For unbalanced speakers, multiplier 512 receives gainsignal Gain1L(k) and multiplier 514 receives gain signal Gain1R(k).Multiplier 512 also receives Filter[In_(est_L)(k)] from Limiter L 502,multiplies Filter[In_(est_L) (k)] with Gain1(k) for balanced speakers(or Gain1L(k) for unbalanced speakers) and outputs the productV_(L_in)(k). Multiplier 514 also receives Filter[In_(est_R)(k)] fromLimiter R 504, multiplies Filter[In_(est_R)(k)] with Gain1(k) forbalanced speakers (or Gain1R(k) for unbalanced speakers) and outputs theproduct V_(R_in)(k).

In a multi-channel system, limiter assembly 220 further includes asecond stage limiter (Limiter 2) 522 that has two voltage inputs, one ofwhich receives V_(L_in)(k) and the other of which receives V_(R_in)(k).Second stage limiter 522 has two resistive value inputs, one of whichreceives Res L(n) and the other of which receives Res R(n). Second stagelimiter 522 has a power input at which Input Power Total(n) is received.The stereo input power (Stereo Input Power(k)) to second stage limiter522 equals V_(L_in)(k)²/Res L(n)+V_(R_in)(k)²/Res R(n), while theavailable power is given by the Input Power Total(n). Hence, the outputof second stage limiter 522, denoted by

${{Gain}2(k)} = {{{MIN}\left( {1,\sqrt{\frac{{Input}{Power}{Total}(n)}{{Stereo}{Input}{Power}(k)}}} \right)}.}$

A left channel multiplier 524 multiplies Gain2(k) with Gain1(k),received from component 506 for balanced speakers (or Gain1L(k) fromLimiter L 502 for unbalanced speakers), the result of which ismultiplied with the left channel audio signal that has been delayed bylook ahead delay 208 by multiplier 236 to generate a left channel audiooutput denoted by Out_(L)(k−Delay). A right channel multiplier 526multiplies Gain2(k) with Gain1(k), received from component 506 forbalanced speakers (or Gain1R(k) from Limiter R 504 for unbalancedspeakers), the result of which is multiplied with the right channelaudio signal that has been delayed by look ahead delay 212 by multiplier238 to generate right channel audio output denoted by Out_(R)(k−Delay).Look ahead delays 208 and 212 ensure that the slow application ofrespective gain signals due to the smoothing filters 502 a and 504 adescribed above do not cause clipping and that the audio is delayed bythe correct amount to compensate for the slow gain. In an example, Delaymay be 128 samples corresponding to a look ahead delay of 2.7 μs foraudio sampling rate=48 KHz.

The two-stage limiter process ensures maximum utilization of audioheadroom in multi-channel system. In a stereo system, the conventionalapproach is to statically divide the total power across the L and Rchannels (typically in equal proportion for balanced speakers). However,a static allocation may result in excess power available in one channel(due to low audio level in that channel), but the excess power cannot beused in the other channel, which, in the meantime, has been reduced inaudio level because it was exceeding the static power allocated in thatchannel. On the other hand, the two-stage limiter need not divide thetotal power; instead it can, as an example, allocate the total poweritself, individually, to both the channels, i.e., it can over-allocatethe power for the first stage limiter. In an example, Voltage LimitL(n), Voltage Limit R(n) and Input Power Total(n) can be such that thepower available for the L channel and the power available for the Rchannel is equal to Input Power Total(n). This way, both channels wouldbe limited only by the total power of the system (instead of a share orfraction of the total power). The possible over allocation of power inthe first stage limiter (e.g., limiters 502 and 504) can be mitigated bythe second stage limiter (e.g., limiter 522) which can output a secondstage Gain2(k) that ensures that the final audio power from L channeland R channel is not greater than Input Power Total(n).

A pilot tone 242 in the form of a signal of a specific frequency andamplitude may be added to Out_(L)(k−Delay) to generate the left channelaudio out signal (Audio Out L), and a pilot tone 244 in the form of asignal of a specific frequency and amplitude may be added toOut_(R)(k−Delay) to generate the right channel audio out signal (AudioOut R), as is known in the art. In an example, the pilot tone may have afrequency of 16 Hz and an amplitude of 90 mV. The frequency andamplitude of pilot tone 242 may be the same or different than those ofpilot tone 244. The pilot tones 242 and 244 are used in the sensor &load resistance measurement components 222 and 224, respectively, todetermine the values of Res L(n) and Res R(n) from Audio Out L and AudioOut R, respectively.

In a single channel system, second gain signal (Gain2(k)) need not becalculated; instead, a single first gain signal (e.g., Gain1L(k) orGain1R(k)) may be used as the input to a single multiplier (236 or 238)to be multiplied with a single delayed audio signal (e.g.,In_(L)(k−delay) or In_(R)(k−delay)), with additional processingcontinuing along either the left or right channel path, to generate asingle Audio Out signal. Thus, components associated with calculation ofGain2(K) may be omitted in a single channel system. Alternatively, in asingle channel system, limiter 522 may operate as a pass-through, i.e.,gain=1, because the first stage limiter 502 or 504 will ensure theproper relationship between power, resistance and voltage.

FIG. 6 is a flow diagram of an example method 600 of operating acontroller, e.g., controller 100, including measurement logic, a batteryvoltage estimator and a current regulator. In operation 602, an analoginput voltage signal is digitized and sampled in each of multiplecalculation periods of time to calculate, for each period of time, amaximum value, an average value, and a minimum value of the inputvoltage signal. In operation 604, an output voltage signal (VBAT(n))representing an estimate of the true input battery voltage is generatedand output for each period of time based on the maximum value for thatperiod of time and the average of multiple maximum values obtainedduring that period of time. In operation 606, a resistance (ESR(n)) isestimated for each period of time based on the minimum value for thecorresponding period of time. In operation 608, a current signal(ILIM(n)) is calculated and output for each period of time based on theoutput voltage signal for that period of time and the estimatedresistance for that period of time.

FIG. 7 is a flow diagram of an example method 700 of operating a firststage limiter, e.g., limiters 502 and 504 and component 506, in atwo-stage limiter assembly in a system with balanced speakers. Inoperation 702, left (L) and right (R) channel filtered audio signals andL and R channel voltage limit signals (Voltage Limit L(n) and VoltageLimit R(n)) are received. In operation 704, the amplitude of each of theL and R channel filtered audio signals is estimated to generateestimates In_(est_L)(k) and In_(est_R)(k), respectively. In operation706, In_(est_L)(k) and In_(est_R)(k) are passed through respectivesmoothing filters to generate filtered estimates Filter[In_(est_L)(k)]and Filter[In_(est_R)(k)], respectively. In operation 708, L and R firststage gain signals Gain1L(k) and Gain1R(k) are calculated as follows:

${{{Gain}1{L(k)}} = {{MIN}\left( {1,\frac{{Voltage}{Limit}{L(n)}}{{Filter}\left\lbrack {{In}_{est_{L}}(k)} \right\rbrack}} \right)}},{and}$$\left( {{{Gain}1{R(k)}} = {{{MIN}\left( {1,\frac{{Voltage}{Limit}{R(n)}}{{Filter}\left\lbrack {{In}_{est_{R}}(k)} \right\rbrack}} \right)}.}} \right.$

In operation 710, the minimum of Gain1L(k) and Gain1R(k) is selected asa common gain Gain1(k).

FIG. 8 is a flow diagram of an example method 800 of operating a secondstage limiter, e.g., limiter 522, in a two-stage limiter assembly in asystem with balanced speakers. In operation 802, Gain1(k), received fromthe first stage limiter, is multiplied with the L and R channel filteredaudio signals, respectively, to generate voltage signals V_(L_in)(k) andV_(R_in)(k), respectively. In operation 804, V_(L_in)(k) and V_(R_in)(k)are input to the second stage limiter, along with Res L(n), Res R(n) andInput Power Total(n). In operation 806, Stereo Input Power(k) iscalculated according to the formula: V_(L_in)(k)²/ResL(n)+V_(R_in)(k)²/Res R(n). In operation 808, Gain2(k), which is theoutput of the second stage limiter, is calculated according to theformula:

${{MIN}\left( {1,\sqrt{\frac{{Input}{Power}{{Total}(n)}}{{Stereo}{Input}{Power}(k)}}} \right)}.$

FIGS. 6, 7 and 8 each depict one possible order of operations employedto control an audio system. Not all operations need necessarily beperformed in the order described. Some operations may be combined into asingle operation. Additional operations and/or alternative operationsmay be performed depending on the particular audio system. For example,in a system with unbalanced speakers, the individual left and rightchannel gains (Gain1L(k) and Gain1R(k)) would be used, instead of acommon gain (Gain1(k)), as described above.

The foregoing describes an intelligent power management solution thatimproves audio performance while managing power supply ripple andpreserving battery life. In various examples, control components ofaudio playback systems may be configured and operated to provideimproved loudness and dynamic range, particularly for audio with a largevariation in content, as well as less distortion. The control techniquesdescribed herein are effective at all battery conditions, especially atlow battery conditions. In addition to improving sound quality, examplesdescribed herein may extend battery life.

In examples, VBAT ripple is taken into consideration while regulatingcurrent drawn from the battery. Even if VBAT is fixed, the ripple at theboost circuit input may cause clipping if the output power requirementexceeds the instantaneous power available at the boost circuit input. Inexamples, optimum current regulation is achieved if the audio power isregulated such that it is less than the available power afterconsidering the ripple in VBAT. In examples, audio power is regulated insuch a way that it is very close to the available power so that theheadroom and loudness is maximized and yet there is no audioclipping/distortion. In examples, to compute the optimum current limit,true battery voltage VBAT is estimated at silence/low ripple condition.In examples, the effect of ESR on ripple (slow change) is compensated byan iterative process; estimated ESR is increased whenever theinstantaneous voltage at the boost circuit input falls below theclipping threshold. Similarly, when such voltage goes above the clippingthreshold, the estimated ESR is decreased at a very slow rate to trackchanges in ESR due to change in system conditions (which is usually avery slow phenomenon). In examples, the effect of the decouplingcapacitor on ripple (which is usually a fast, frequency dependentphenomenon) is compensated by a filter. As the frequency of the audioinput changes, the filter allows frequency dependent power/currentregulation to maximize the audio loudness without sacrificing audioquality (i.e., without clipping or distortion).

The term “coupled” is used throughout the specification. The term andderivatives thereof may cover connections, communications, or signalpaths that enable a functional relationship consistent with thisdescription. For example, if device A provides a signal to controldevice B to perform an action, in a first example device A is coupled todevice B, or in a second example device A is coupled to device B throughintervening component C if intervening component C does notsubstantially alter the functional relationship between device A anddevice B such that device B is controlled by device A via the controlsignal provided by device A.

A device that is “configured to” perform a task or function may beconfigured (e.g., programmed and/or hardwired) at a time ofmanufacturing by a manufacturer to perform the function and/or may beconfigurable (or re-configurable) by a user after manufacturing toperform the function and/or other additional or alternative functions.The configuring may be through firmware and/or software programming ofthe device, through a construction and/or layout of hardware componentsand interconnections of the device, or a combination thereof.

As used herein, the term “terminal” may refer to a node,interconnection, pin and/or a lead. Unless specifically stated to thecontrary, these terms are generally used to mean an interconnectionbetween or a terminus of a device element, a circuit element, anintegrated circuit, a device or other electronics or semiconductorcomponent.

A circuit or device that is described herein as including certaincomponents may instead be adapted to be coupled to those components toform the described circuitry or device. For example, a structuredescribed as including one or more semiconductor elements (such astransistors), one or more passive elements (such as resistors,capacitors, and/or inductors), and/or one or more sources (such asvoltage and/or current sources) may instead include only thesemiconductor elements within a single physical device (e.g., asemiconductor die and/or integrated circuit (IC) package) and may beadapted to be coupled to at least some of the passive elements and/orthe sources to form the described structure either at a time ofmanufacture or after a time of manufacture, for example, by an end-userand/or a third-party.

Circuits described herein are reconfigurable to include the replacedcomponents to provide functionality at least partially similar tofunctionality available prior to the component replacement. Componentsshown as resistors, unless otherwise stated, are generallyrepresentative of any one or more elements coupled in series and/orparallel to provide an amount of impedance represented by the shownresistor. For example, a resistor or capacitor shown and describedherein as a single component may instead be multiple resistors orcapacitors, respectively, coupled in parallel between the same nodes.For example, a resistor or capacitor shown and described herein as asingle component may instead be multiple resistors or capacitors,respectively, coupled in series between the same two nodes as the singleresistor or capacitor.

Uses of the phrase “ground” in the foregoing description include achassis ground, an Earth ground, a floating ground, a virtual ground, adigital ground, a common ground, and/or any other form of groundconnection applicable to, or suitable for, the teachings of thisdescription. Unless otherwise stated, “about,” “approximately,” or“substantially” preceding a value means +/−10 percent of the statedvalue.

Modifications of the described examples are possible, as are otherexamples, within the scope of the claims. For example, an average ofmultiple Min VBAT(n) values may be used to adjust ESR. Moreover, whileexamples shown are directed to stereo and mono solutions, the teachingsmay be applied to multi-channel systems in which the number of channelsis greater than 2. In such case, power threshold calculator 214 outputsa Voltage Limit signal for each channel, limiter assembly 220 receives afiltered audio signal for each channel, and the first stage of limiterassembly 220 includes a limiter for each channel. Also, the teachingsmay be applied to boost circuits, which, in addition to having controlon the peak current limit, may also have control on the duty cycle ofcharge and discharge.

Features described herein may be applied in other environments andapplications consistent with the teachings provided.

What is claimed is:
 1. A controller comprising: a voltage estimatorhaving inputs to receive multiple input voltage values obtained from aninput voltage signal and an output at which a voltage signal (estimatedVBAT) representing an estimate of the true input voltage signal isoutput; and a current regulator having a first input to receive one ofthe multiple input voltage values, a second input coupled to the outputof the voltage estimator to receive the estimated VBAT, and an output atwhich a calculated current limit signal is output, the current regulatortracking an estimated resistance and controlling the estimatedresistance to be a specific value based on the input voltage valuereceived by the current regulator, the current regulator calculating thecurrent limit signal based on the estimated VBAT, a clipping voltagethreshold and the specific value of the estimated resistance.
 2. Thecontroller of claim 1, comprising: measurement logic having an input toreceive the input voltage signal, the measurement logic being configuredto sample the input voltage signal in each of multiple calculationintervals and, for each calculation interval, generate a maximum inputvoltage value (maximum value), an average input voltage value (averagevalue), a minimum input voltage value (minimum value), wherein theinputs of the voltage estimator to receive the multiple input voltagevalues include a first input to receive the maximum value for eachcalculation interval, and a second input to receive the average valuefor each calculation interval, the voltage estimator being configured togenerate an intermediate value based on the difference between themaximum value and the average value for that calculation interval and anaverage of multiple intermediate values, and wherein the one of themultiple input voltage values received at the first input of the currentregulator is the minimum value for each calculation interval.
 3. Thecontroller of claim 2, wherein, for each calculation interval, thecurrent regulator increases the value of the estimated resistance whenthe minimum value falls below the clipping voltage threshold, decreasesthe value of the estimated resistance when the minimum is at or abovethe clipping voltage threshold and an audio signal is compressed, andmaintains the value of the estimated resistance when the minimum valueis at or above the clipping voltage threshold and the audio signal isnot compressed.
 4. The controller of claim 2, wherein, for eachcalculation interval, the voltage estimator compares the differencebetween the maximum value and the average value for that calculationinterval with a silence voltage threshold to generate the intermediatevalue, and wherein the voltage estimator estimates VBAT based on theaverage of multiple intermediate values.
 5. The controller of claim 4,wherein, for each calculation interval, the voltage estimator determinesthe intermediate value to be equal to the maximum value when thedifference is less than the silence voltage threshold.
 6. The controllerof claim 4, wherein, for each calculation interval, the voltageestimator determines the intermediate value based on the intermediatevalue of a previous calculation interval when the difference is greaterthan or equal to the silence voltage threshold.
 7. The controller ofclaim 1, wherein the estimated resistance represents a combination ofresistance in a power supply from which the input voltage signal isdirectly or indirectly obtained and parasitic resistance.
 8. An audiosystem comprising: a voltage estimator that generates a voltage signal(estimated VBAT) representing an estimate of true voltage input to theaudio system based on multiple input voltage values; a current regulatorthat tracks an estimated resistance and controls the estimatedresistance to be a specific value based on one of the multiple inputvoltage values that is received by the current regulator, the currentregulator calculating a current limit signal based on the estimatedVBAT, a clipping voltage threshold and the specific value of theestimated resistance; a filter disposed in an audio signal path, thefilter having an output at which a filtered audio signal is output; andan audio limiter having a first input at which the filtered audio signalis received, a second input at which a voltage limit signal is received,a third input at which a load resistance value determined from an audiooutput signal is received, and a fourth input at which an input powersignal is received.
 9. The audio system of claim 8, wherein the audiolimiter controls power allocated for audio playback based on thefiltered audio signal, the voltage limit signal, the load resistancevalue, and the input power signal.
 10. The audio system of claim 8,further comprising: a boost circuit having a voltage input at which adevice voltage is received and a current input at which the currentlimit signal is received; and an inductor coupled to the voltage inputof the boost circuit, the inductor drawing current from a battery,wherein the peak of the current drawn from the battery is limited by thecurrent limit signal.
 11. The audio system of claim 8, furthercomprising: a power threshold calculator having a voltage input at whichthe estimated VBAT is received, a current input at which the currentlimit signal is received, and a resistance input at which the loadresistance value is received, the power threshold calculator calculatingthe voltage limit signal based on the estimated VBAT, the current limitsignal, and the load resistance value.
 12. The audio system of claim 11,wherein the power threshold calculator calculates a scaled input powerbased on the estimated VBAT, the current limit signal and a scalefactor, and then calculates the voltage limit signal based on the scaledinput power and the load resistance value.
 13. The audio system of claim12, wherein the power threshold calculator includes an input powercalculator that calculates an input power total signal based on thescaled input power.
 14. A method comprising: measuring an input voltagesignal to a controller of an audio system in each of multiple periods oftime to calculate, for each period of time, a maximum value of the inputvoltage signal (maximum value), an average value of the input voltagesignal (average value), and a minimum value of the input voltage signal(minimum value); outputting, by the controller, an output voltage foreach period of time representing an estimate of the true value of theinput voltage signal based on the maximum value for that period of timeand the average of multiple maximum values sampled during that period oftime; estimating a resistance for each period of time based on theminimum value for that period of time; and calculating an output currentof the controller for each period of time based on the output voltagefor that period of time and the estimated resistance for that period oftime.
 15. The method of claim 14, wherein the output voltage for eachperiod of time is determined based on an intermediate value that isdetermined based on a difference between the maximum value for thatperiod of time and the average value obtained during that period of timecompared to a silence voltage threshold.
 16. The method of claim 15,wherein, for each period of time, the output voltage is determined basedon an average of multiple intermediate values, one from a present periodof time and each of the others from a respective previous period oftime.
 17. The method of claim 15, wherein, for each period of time, whenthe comparison indicates that the difference is less than the silencevoltage threshold, the intermediate value is determined to be equal tomaximum value obtained during that period of time.
 18. The method ofclaim 15, wherein, for each period of time, when the comparisonindicates that the difference is greater than or equal to the silencevoltage threshold, the intermediate value is maintained the same as inan immediate previous period of time.
 19. The method of claim 14,wherein the estimating of the resistance for each period of timeincludes comparing the minimum value for that period of time to aclipping voltage threshold.
 20. The method of claim 19, wherein, foreach period of time, when the comparing indicates that the minimum valueis less than the clipping voltage threshold, the estimated resistance isdetermined to be the sum of the estimated resistance for an immediatelyprevious period of time and a first amount of resistance.
 21. The methodof claim 19, wherein, for each period of time, when the comparingindicates that the minimum value is greater than or equal to theclipping voltage threshold, the method further comprises determiningwhether audio is under compression.
 22. The method of claim 21, wherein,for each period of time, when it is determined that the audio is undercompression, the estimated resistance is determined to be the differencebetween the estimated resistance for an immediately previous period oftime and a second amount of resistance, and when it determined that theaudio is not under compression, the estimated resistance is determinedto be the estimated resistance for an immediately previous period oftime.
 23. The method of claim 14, wherein, for each period of time, theoutput current is calculated to be the difference between the outputvoltage and a clipping voltage threshold divided by the estimatedresistance.
 24. The method of claim 14, further comprising: filtering aninput audio signal to compensate for capacitive ripple.
 25. An audiosystem comprising: a limiter having a filtered audio input at which afiltered audio signal is received, a voltage limit input at which avoltage limit signal is received, and a gain output at which a gainsignal, calculated based on the filtered audio signal and the voltagelimit signal, is output, wherein the limiter includes a smoothing filterconfigured to estimate the amplitude of the filtered audio signal andoutput a signal representing a smoothened estimate of the amplitude ofthe filtered audio signal (smoothened estimate signal).
 26. The audiosystem of claim 25, wherein the limiter includes a multiplier configuredto calculate a product signal representing the product of the gainsignal and the smoothened estimate signal.
 27. The audio system of claim26, wherein the limiter has a resistance input at which a resistancevalue signal is received and a total available power input at which atotal available power signal is received, the limiter being configuredto calculate a total gain signal based on the resistance value signal,the total available power signal and the product signal.
 28. The audiosystem of claim 25, further comprising a filter having a first audiosignal input and a filtered audio output coupled to the filtered audioinput of the limiter.
 29. The audio system of claim 28, furthercomprising a look ahead delay having a second audio signal input and adelayed audio signal output.
 30. The audio system of claim 25, wherein:the filtered audio input includes left and right channel filtered audioinputs at which left and right channel filtered audio signals arereceived, respectively, the voltage limit input includes left and rightchannel voltage limit inputs at which left and right channel voltagelimit signals are received, respectively, the gain output includes leftand right channel gain outputs at which left and right channel gainsignals are output respectively, the left channel gain output signalbeing calculated based on the left channel filtered audio signal and theleft channel voltage limit signal, the right channel gain output signalbeing calculated based on the right channel filtered audio signal andthe left channel voltage limit signal, and the smoothing filter includesleft and right channel smoothing filters configured to estimate theamplitudes of the left channel and right channel filtered audio signals,respectively, and output signals representing smoothened estimates ofthe amplitudes of the left and right channel filtered audio signals. 31.The audio system of claim 30, wherein the limiter includes left andright channel multipliers configured to calculate left and right productsignals, the left product signal representing the product of the leftchannel gain signal and the left channel smoothened estimate signal, andthe right product signal representing the product of the right channelgain signal and the right channel smoothened estimate signal.
 32. Theaudio system of claim 30, wherein the limiter includes left channel andright channel resistance inputs at which left and right channelresistance value signals are received, respectively, and a totalavailable power input at which a total available power signal isreceived, the limiter being configured to calculate a total gain signalbased on the left and right channel resistance value signals, the totalavailable power signal and the left and right product signals.
 33. Theaudio system of claim 30, further comprising: a left channel filterhaving a left channel audio signal input and a left channel filteredaudio output coupled to the left channel filtered audio input of thelimiter; and a right channel filter having a right channel audio signalinput and a right channel filtered audio output coupled to the rightchannel filtered audio input of the limiter.
 34. The audio system ofclaim 30, further comprising: a left channel look ahead delay having aleft channel audio signal input and a left channel delayed audio signaloutput; and a right channel look ahead delay having a right channelaudio signal input and a right channel delayed audio signal output.